A leading opposition challenger to president Joseph Kabila has called for the elections to be annulled, accusing authorities of systematic fraud.

Vital Kamerhe, a former government minister, said ballots had been marked in advance of the poll in favour of Kabila, and some voters had been prevented from entering polling stations during Monday's chaotic polls.

"There can be no doubt as to the scale of the fraud, deliberately planned by those in power with the connivance of the national election commission," Kamerhe wrote in a letter to Kabila, the election commission and international bodies.

"Police chased witnesses from polling stations before counting could start," he added, citing reports by international observers and others that security forces took control of voting stations in Kinshasa. "These elections must quite simply be annulled."

Three other presidential candidates urged the Congolese not to accept any results from the vote, saying widespread technical problems and fraud meant they would not be credible.

But the demands were rejected by Congo's national election commission, which insisted the process had mostly gone smoothly.

"The majority of polling stations opened," said Daniel Ngoy Mulunda, head of the commission. "Violence happened in very few places and counting is going on, so I don't see why we should annul the elections."

He said of the complaining candidates: "They voted themselves. The result will be announced. If they win, they win, if they lose, they lose."

Mulunda insisted that 99.2% of polling stations had been working, with problems reported at only 485 from a countrywide total of 63,865.

But he also announced that voting would spill over to a third day. A small number of remaining polling stations would open from 6am to 5pm on Wednesday, he said, but then "all election operations will close tomorrow".

Fresh ballot papers were being flown in from South Africa after a stock was burned in an attack on a delivery truck in Lubumbashi.

Mulunda admitted there had been some mistakes in deploying materials but responding to the fraud allegations, he said: "We need people to bring in these famous ballot papers so we can test them ourselves."

Completed ballot papers will be taken to counting centres in vehicles escorted by UN peacekeepers, official witnesses and journalists, but not the national army, he added.